Uncovering redundant fields in TypeScript and detecting errors through type inference

Question

Uncovering redundant fields in TypeScript and detecting errors through type inference

Encountering an unusual edge case with the TS compiler regarding type inference. Surprisingly, the code snippet below (with commented lines intact) should trigger a compile error, but it doesn't.

interface IReturned {
    theField?: string;
}
interface IFactory {
    (): IReturned;
}
var factory : IFactory = function () /* : IReturned */{
    return {
        BROKEN: 'ERROR'
    }
};
var instance = factory();
// instance.BROKEN;

Ideally, the return type of the factory function should be inferred as IReturned, making it invalid to return an object literal with extra fields, such as setting the "BROKEN" property.

If the function is explicitly annotated with IReturned as its return type, this behavior is enforced. The type of the "instance" variable is correctly inferred, and accessing the "BROKEN" field below triggers an error.

Is there something missing here? Is there a workaround to achieve the desired result without redundant type definitions?

typescript type-inference

Answer 1

Answer №1

The approval is based on its compatibility.

Consider a scenario where you create an interface called IRet2 which extends IReturned and introduces a new field called BROKEN. Since IRet2 extends IReturned, it is acceptable to return an object of type IRet2 in situations where an IReturned is expected:

interface IReturned {
    theField?: string;
}
interface IRet2 extends IReturned {
    BROKEN: string;
}
interface IFactory {
    (): IReturned;
}
var factory : IFactory = function (): IRet2 {
    return {
        BROKEN: 'ERROR'
    }
};
var instance = factory();

Your implementation adheres to the same concept, but with a deduced type that inherently extends IReturned.

Answer 2

The approval is based on its compatibility.

Consider a scenario where you create an interface called IRet2 which extends IReturned and introduces a new field called BROKEN. Since IRet2 extends IReturned, it is acceptable to return an object of type IRet2 in situations where an IReturned is expected:

interface IReturned {
    theField?: string;
}
interface IRet2 extends IReturned {
    BROKEN: string;
}
interface IFactory {
    (): IReturned;
}
var factory : IFactory = function (): IRet2 {
    return {
        BROKEN: 'ERROR'
    }
};
var instance = factory();

Your implementation adheres to the same concept, but with a deduced type that inherently extends IReturned.

Uncovering redundant fields in TypeScript and detecting errors through type inference

Answer №1

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